For power converters represented in hybrid cars, electric vehicles, and the like, semiconductor modules are widely used. Semiconductor modules that constitute a controller for energy conservation comprise power semiconductor devices in order to control large currents.
Such power semiconductor devices are generating more and more heat when controlling large currents. In particular, the amount of heat generation increases significantly as the sizes of power semiconductor devices are reduced and output of power semiconductor devices is increased. Therefore, with a semiconductor module comprising a plurality of power semiconductor devices, the methods of cooling the power semiconductor devices pose a major problem.
Conventionally, a liquid-cooled cooler (hereinafter, also referred to as a “cooling apparatus”) is generally used in a semiconductor module in order to enhance cooling efficiency of the semiconductor module. With liquid-cooled cooling apparatuses which circulate a coolant, various designs have been implemented in order to enhance cooling efficiency such as increasing coolant flow rate, forming radiating fins (cooling body) in a shape with a high heat transfer rate, and increasing thermal conductivity of a material that constitutes the fins.
However, increasing a coolant flow rate to the cooling apparatus or adopting a fin shape with a high heat transfer rate is likely to cause inconveniences such as increased pressure loss of a coolant flowing inside the apparatus. In particular, with a cooling apparatus that cools a large number of power semiconductor devices using a plurality of heat sinks, when using a serpentine flow channel which connects a plurality of flow channels in series, there is a significant increase in pressure loss. A configuration which enhances cooling efficiency with a low coolant flow rate is considered ideal to reduce such pressure loss. While this can be achieved by, for example, increasing thermal conductivity of the fin material, adopting a new fin material may result in increasing overall apparatus cost.
Cooling apparatuses which attempt to reduce pressure loss of a coolant while maintaining cooling performance are being considered in which a coolant introduction flow channel for introducing the coolant and a coolant discharge flow channel for discharging the coolant are arranged parallel to each other and a plurality of heat sinks is arranged between the coolant introduction flow channel and the coolant discharge flow channel in a coolant circulation direction that is approximately perpendicular to the coolant introduction flow channel and the coolant discharge flow channel (refer to Patent Documents 1 to 7). In this case, configuring the heat sinks so that the coolant flows in parallel between respective fins increases cooling performance per pressure loss. In addition, by varying pressure loss per unit flow channel in accordance with a distance of a fin from a semiconductor device, the pressure loss of the coolant in the flow channel as a whole can be reduced and cooling efficiency can be improved (refer to Patent Document 5).
Furthermore, Patent Document 6 describes a liquid-cooled cooling apparatus in which an entire rear sidewall of a casing is smoothly inclined toward the front from a right sidewall side to a left sidewall side, and a flow channel cross-sectional area of an inlet header section decreases from a coolant inlet side to the left sidewall side (refer to paragraphs [0024], [0031] and FIG. 2).
Moreover, Patent Document 3 describes a liquid-cooled cooling apparatus in which flow channels that introduce and discharge a coolant are arranged on a same side surface of a module and the respective flow channels with constant cross-sectional areas are arranged perpendicular to fins (refer to FIG. 1).    Patent Document 1: Japanese Patent Application Laid-open No. 2001-35981    Patent Document 2: Japanese Patent Application Laid-open No. 2007-12722    Patent Document 3: Japanese Patent Application Laid-open No. 2008-205371    Patent Document 4: Japanese Patent Application Laid-open No. 2008-251932    Patent Document 5: Japanese Patent Application Laid-open No. 2006-80211    Patent Document 6: Japanese Patent Application Laid-open No. 2009-231677    Patent Document 7: Japanese Patent Application Laid-open No. 2006-295178
However, with previous cooling techniques, a disproportionate flow distribution where a coolant flows in an uneven manner is created due to shapes of heat sinks or coolant flow channels, arrangement of heater elements, shapes of an inlet and an outlet of a coolant, or the like. With conventional cooling apparatuses, such a disproportionate flow distribution causes unevenness in cooling performance. Therefore, it is difficult to achieve cooling performance that is uniform and stable throughout the entire cooling apparatus. In addition, since inconveniences occur such as a significant rise in heating temperature of only a semiconductor device arranged at a position directly opposite to a side of a coolant outlet, there are problems of reduced device life, greater susceptibility to failure, or the like.
Moreover, when a flow channel cross-sectional area of an inlet header section decreases in a direction in which the flow channel extends as is the case of the cooling apparatuses disclosed in Patent Documents 6 and 7, although flow rate distribution tends to be improved, the improvement does not extend to resolving the problem of increased temperature in a vicinity of a coolant inlet.
The present invention has been made in consideration of the above, and an object of the present invention is to provide a semiconductor module cooler capable of resolving a disproportionate flow that occurs in a coolant flow channel, effectively cooling a semiconductor device arranged on an outer surface of the cooler, and reliably preventing malfunction or destruction attributable to heat generation by the semiconductor device.